1. Field of the Invention
The present invention relates to a data memory device having an Exclusive-OR function and a method of reading the stored data therefrom for retrieving or answering queries relating to identity/non-identity, which are required for database processing, image processing, word processing or the like.
2. Description of the Related Art
A functional memory having both a storage element and an functional circuit serves not only to store at least one data but also to simultaneously perform various sorts of computation. The functional memory is used, for example, as associative memory, logic-in-memory, distributed logic memory, etc. Furthermore, the associative memory by way of example is used as content addressable memory, data address memory, search memory, etc. It is generally recognized that this kind of functional memory is proposed for the first time in "The Cryotron Catalog Memory System" by Slade and McMahon published in 1956. There have been further proposed a data storage by utilizing magnetism of magnetic core (1961) or magnetic bubbles, as well as a data memory in which a semiconductor flip-flo
circuit is combined with an identity circuit (E. S. Lee; 1963). However, these memories have not been mass produced due to various reasons. For example, in the case of magnetic core, an coincident-current selection control is required which makes it difficult to control the destructive readout and non-destructive readout and thus necessitates the stored data to be read in a destructive readout mode in which the retrieval rate of the data is relatively low.
The functional memory can be used not only in small scale applications such as address conversion in a virtual memory, memory mapping in a cash memory, conversion of symbolic address, but also retrieve or answer queries relating to identity/non-identity, which are required for database processing, image processing, word processing or the like. However, despite the long history and practical importance of the functional memory, there has been no marked breakthrough matched for large scale applications, beyond a small scale memory for limited applications.
On the other hand, since a main memory is highly important element for the development of computers, research and development in respect of semiconductor RAM (Random Access Memory) have been devoted. As a result of rapid progress in the RAM technology, which became particularly drastic upon appearance of DRAM (Dynamic Random Access Memory), it can be composed of a single memory cell in the form of a capacitor storing charges and a single transistor, and can be made extremely compact to occupy a minimized area.
Since the functional memory comprises a memory cell and an functional element, as mentioned above, and the memory cell is typically composed of at least one SRAM (Static Random Access Memory) and/or DRAM as used in a main memory, the elements of a functional memory is always larger in number than those of SRAM or DRAM and thus occupy a larger area. For such reason, it has been considered more general and economical to perform data processing by using, instead of a dedicated functional memory, a main memory in combination with an functional circuit with the sacrifice of the processing speed. Such recognition in the art has been a prejudicial factor to the development of advanced functional memories.
Memories having a small number of elements and a large storing capacity are being under development, based on flush memory technology or memory elements similar to flush memory but using a plurality of threshold values. In this case, however, it is inevitable that the number of the elements is twice as much as that of DRAM, besides that the data writing rate is insufficient. As a result, such memories still do not have high speed and large storing capacity as comparable to those in DRAM or SRAM.
The arrangement of functional memory cells can be classified into WPBP (Word Parallel Bit Parallel) mode and WPBS (Word Parallel Bit Serial) mode. While the WPBP mode is capable of performing all calculations based on a single instruction, the calculations are practically divided in terms of reading out of the calculated results or in view of power consumption, following the progressive integration of the memory cells.
While various functional memories have been so far proposed, as explained above, it is recognized that a further progress in the integration of functional memories requires (i) a simple structure similar to semiconductor memory which is comprised of a single transistor and a single memory cell, (ii) a WPBS operation similar to the WPBP mode which allows parallel operations within allowable data transfer rate and power consumption, and (iii) a functional limitation to simple calculations for retrieval of identity/non-identity or Exclusive-OR operation with a modulo-2 adding operation.